You need a good pipe and header. Mac’s pipes and headers are what I use. The #1050 Quiet pipe, #1250 Muffled and the 1150 tuned (noisy) I have all tried and all work well!
To my biggest surprise the quiet pipe rendered the most speed, at least measured on the ground. The best average pipe setting was 9 ½” from center of piston to the large diameter on the pipe. Cutting this ¼” shorter may let the engine stage even higher in the air at the cost of smooth transition, so try this later on if you dare.
The highest speed measured was 24,500 RPM (on ground) spinning a APC carbon (LP 74825C) 7.4X 8.25 on Power Master 30%Heli fuel. OS #8 plug. I tried 15% and RPM dropped to 23K and I tried Wild cat 45% and it dropped to 23K, I think the compression is too high for it. Run with 15% or 30% nitro, it’s plenty fast. An APC 7.8X7 carbon should turn 23,900 with the same set up.

With the header/pipe set longer it should turn a pylon 8.75x9.5NN or 8.8x9.25 at 20,000RPM. Don't use a prop with less load or sport prop, IT WILL THROW A BLADE! Stay inside rpm limits and always keep anything you care about out of the prop arc. Ream out the prop to fit the short shaft addapter (Tru Turn part number TT-0141-A) that you will need for all the props.

You have three header choices’, strait, 15* and fuse top.
The fuse top could help clear the fuel tank however the header length is so short that you need to cut both header and pipe. I made two small aluminum blocks that allow me to rotate the carb so the needle can clear the mounting rails with out too much cutting. A good Idea is to buid a test stand for break-in and testing. Note center of fuel tank is centered with carb. Don't run it too rich at break-in, just wastes fuel and could hurt the con rod. Engines need some heat to seat.
I start with Power master Heli 15% /20, I like the extra oil.